Cellular Signalling 27 (2015) 673–682

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Cellular Signalling

j ourna l homepage: www.e lsev ie r .com/ locate /ce l l s ig

Review

Sirtuin 7 in cell proliferation, stress and disease: Rise of theSeventh Sirtuin!

Shashi Kiran a,⁎, Tarique Anwar a, Manjari Kiran c, Gayatri Ramakrishna b,⁎⁎a Laboratory of Cancer Biology, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, Telangana 500001, Indiab Laboratory of Cancer Cell Biology, Department of Research, Institute of Liver and Biliary Sciences, Delhi 110070, Indiac Laboratory of Computational Biology, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, Telangana 500001, India

⁎ Corresponding author.⁎⁎ Correspondence to: G. Ramakrishna, Laboratory of Ca

http://dx.doi.org/10.1016/j.cellsig.2014.11.0260898-6568/© 2014 Elsevier Inc. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 November 2014Accepted 21 November 2014Available online 27 November 2014

Keywords:SIRT7SirtuinStressCancerHepatic lipid metabolism

Sirtuin 7 is a member of the sirtuin family of proteins. Sirtuins were originally discovered in yeast for its role inprolonging replicative lifespan. Until recently SIRT7 happened to be the least studied sirtuin of the sevenmammalian sirtuins. However, a number of recent breakthrough reports have provided significant clarity toSIRT7 biology. SIRT7 is now seen as a vital regulator of rRNA and protein synthesis formaintenance of normal cel-lular homeostasis. Proteins like p53, H3K18, PAF53, NPM1 and GABP-β1 are the known substrates for thedeacetylase activity of SIRT7, thereby making it a key mediator of many cellular activities. Studies usingin vitro based assays and also knockout mice have revealed a role of SIRT7 in certain disease pathologies aswell. High expression of SIRT7 has been reported in few cancer types and is steadily propelling SIRT7 towardsan oncogene status. The role of SIRT7 as a pro-survival adaptor molecule in conditions of cellular stress has re-cently emerged in view of the fact that SIRT7 can regulate molecules like HIF and IRE1α. Additionally, SIRT7plays a key role in maintenance of the epigenome as it caused the deacetylation of histone (H3K18) and globalproteomics studies have shown its interaction with many chromatin remodelling complexes such as B-WICHand other proteins. Lately, the role of SIRT7 in hepatic lipid metabolism has been debated. This review attemptsto summarize these recent findings and present the role of SIRT7 as an important cellular regulator.

© 2014 Elsevier Inc. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6742. Genomic organization of SIRT7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6743. Catalytic activity of SIRT7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6744. Localization and nucleolar targeting of SIRT7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6755. Expression pattern of SIRT7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6766. Role of SIRT7 in rDNA transcription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6767. SIRT7 and protein synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6768. SIRT7 in cellular stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676

8.1. Hypoxia induced stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6778.2. Low glucose stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6788.3. Endoplasmic reticulum stress (ER-stress) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6788.4. Genomic stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678

9. SIRT7 and chromatin remodelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67810. Regulation of SIRT7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67811. SIRT7 in disease and cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679

11.1. SIRT7 and cardiac health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67911.2. SIRT7 in hepatic steatosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67911.3. Role of SIRT7 in cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680

12. SIRT7 in ageing and senescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680

ncer Cell Biology, Department of Research, Institute of Liver and Biliary Sciences, Delhi, 110070, India.

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13. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682

1. Introduction

Sirtuins are a class of deacetylase initially discovered in yeast as Sir2protein. They are unique among the family of deacetylases in the factthat they require NAD+ for their catalytic activity thereby linking theircatalytic activities to cellular energetics. Sirtuins have been an intensefield of study during the last decade after they were found to extendlifespan in lower organisms like yeast, Drosophila and Caenorhabditiselegans [1–3]. In mammals sirtuins have emerged as a major metabolicregulator rather than having a direct effect on increasing lifespan.Amongmammalian sirtuins, only SIRT6 has been directly linked tomam-malian ageing [4]. Instead, mammalian sirtuins have been linked to anumber of cellular and metabolic processes and have been found tobe important for a healthy ageing [5–8]. Mammalian sirtuin familyconsists of seven different sirtuins (SIRT1 - SIRT7) characterised by aNAD+binding domain and a catalytic domain. These are localized tothree different subcellular compartments— SIRT1 andSIRT6 areprimarilynuclear sirtuins, SIRT2 is cytoplasmic, SIRT3, 4 and 5 are mitochondrialsirtuins and SIRT7 is primarily localized to nucleolus [9]. Of the sevensirtuins, SIRT1 has been the most studied sirtuin primarily because of itsclose similarity to the yeast Sir2 protein and its strong catalytic activity.A number of substrates are nowknown for SIRT1which play a role in var-ious cellular andmetabolic processes. SIRT3 has been another thoroughlyinvestigated mammalian sirtuins and has emerged as the major mito-chondrial deacetylase [10–12]. The role of SIRT6 has largely emerged inmaintenance of genome integrity and DNA damage [13–15]. The role ofSIRT2 along with SIRT1 has emerged primarily in neurodegenerative dis-eases [16]. SIRT4 and SIRT5 act on enzymes of several importantmetabol-ic processes [17–19]. Of all the mammalian sirtuins SIRT7 had receivedleast attention in the initial years of sirtuin research and very little wasknown about its cellular functions until recently. However, in the lasttwo years, a number of reports have highlighted the role of SIRT7 invarious cellular processes. Apart from its role in rDNA transcription, the

Fig. 1. Genomic organisation of SIRT7. (A,B) SIRT7 gene occupies 6256 base pairs at telomeric(C) SIRT7 transcript after splicing is 1743 bp long and translates into 400 amino acid long proteclear localization signal; NoLS, Nucleolar localization signal) are important functional domains oof all Sirtuins (amino acid-188 for SIRT7).

role of SIRT7 has emerged in protein synthesis, chromatin remodelling,cellular survival and lipid metabolism. The recent findings on SIRT7functioning are elaborated in details below.

2. Genomic organization of SIRT7

SIRT7 is a single copy gene located at the subtelomeric region onchromosome 17 (17q25.3) and the genomic sequence spans a 6.2 kbregion [20]. SIRT7 gene encodes 10 exons, exon 4 being the shortest(71 bp long) and exon 7 the longest (237 bp long) (Fig. 1). The openreading frame of human SIRT7 is 1203 bp long which encodes a400 amino acid long protein. The molecular weight of this protein isabout 44.9 kDa and the isoelectric point is 9.8. SIRT7 bears only 21% iden-tity to yeast Sir2, the founder of sirtuin family of proteins (least among thesevenmammalian sirtuins). Among the sevenmammalian sirtuins, SIRT7bears the closest similarity (56% identity) to the protein sequence of SIRT6and bears 39% identity to themost studied human sirtuin, SIRT1. The con-served catalytic domain is between 90 and 331 aa (encompassing exons 3to 9) in the SIRT7 sequence [20].

3. Catalytic activity of SIRT7

Like all sirtuins, the histidine residue required for NAD+ binding inthe catalytic domain of sirtuins is well conserved at position 188 ofSIRT7 amino acid sequence. However, sirtuin 7 does not exhibit a strongdeacetylase activity on commonly used substrates. Michishita et al. [9]found no deacetylase activity for SIRT7 in relation to a p53 peptidecontaining acetylated K382 of p53 as well as a histone H4 peptidecontaining acetylated K16 residue [9]. Later, Vakhrusheva et al. reportedincreased levels of acetylated K382 on p53 in SIRT7 knockout mice.However, the levels of p53 were also found to be elevated in thesemice. They also demonstrated an in vitro deacetylase activity ofSIRT7 at efficiencies equivalent to SIRT1 using a p53-K382/diAc peptide

end of chromosome 17 which encodes for 10 exons (red box) and 9 introns (black box) .in. (D) The catalytic NAD+ binding domain (green line) and localisation signals (NLS, Nu-f SIRT7 Black line indicates the conserved Histidine residue important for catalytic activity

675S. Kiran et al. / Cellular Signalling 27 (2015) 673–682

as a substrate [21]. In contrast, Barber et al. noted an absence ofdeacetylase activity of SIRT7 with respect to p53 using p53 K382Ac as asubstrate [22]. Kimet al. [35] in turn reported an increaseddeacetylase ac-tivity against p53 in nuclear immuno-precipitates of Hep3B cells using aSIRT7 antibody. Thus, there have been contrasting reports regarding thedeacetylase activity of SIRT7 against p53 and the reason for these variableobservations is open for further investigation [35].

Barber et al. [22] reported H3K18 deacetylation activity to be associ-ated with SIRT7. SIRT7 mediated H3K18 deacetylation was found tobe NAD+ dependent and was absent for catalytically inactive form ofSIRT7 (SIRT7-H188Y). Also unlike SIRT1 and other histone deacetylasesthe deacetylase activity of SIRT7 was limited to H3K18 acetylation butnot on other acetylated histone peptides thereby establishing SIRT7 asa specific H3K18 deacetylase. SIRT7 mediated H3K18 deacetylation didnot change global H3K18 acetylation levels but was limited to thepromoter of a certain set of genes. This target set of promoters wasdefined by binding sites of ELK4 as SIRT7 lacks a DNA binding domainin itself and hence binds to chromatin through ELK4 [22]. SIRT7mediat-ed H3K18 deacetylation was also observed by Karim et al. [23]. Addi-tionally, they reported that this deacetylase activity was inhibited byMyb-binding protein 1a (Mybbp1a) by specifically binding to SIRT7[23].

Another SIRT7 substrate is PAF53, a subunit of RNA polymerase Icomplex which is required for the recruitment of RNA polymerase I torDNA promoters. SIRT7 maintains PAF53 in deacetylated state, a staterequired for it to be chromatin bound during rDNA transcription [24].Acetylation of PAF53 by CBP in turn reduces its chromatin occupancy.Recently, nucleophosmin (NPM1) was found to be a substrate for thecatalytic activity of SIRT7 [25]. An acetylation level of exogenouslyexpressed NPM1was decreased upon the overexpression of SIRT7. How-ever, the deacetylase activity of SIRT7 could only be detected when P300(a known acetyl transferase for NPM1) was also co-transfected withSIRT7 [25]. Another recently identified substrate for SIRT7 is GABP-β1 [26]. SIRT7 overexpression greatly reduced acetylation levels ofGABP-β1 while it was hyperacetylated in SIRT7 knockout cells.SIRT7 deacetylated GABP-β1 at three different lysine residues (K69,K340 and K369) to maintain its dimerization capability [26].

SIRT7 depletion did not change the global acetylation levels of eithernucleolar or nuclear proteomes indicating that SIRT7 deacetylase activ-ity is specific to a limited set of proteins [27]. Also, some proteins whichwere found to interact with SIRT7 did not undergo SIRT7 mediateddeacetylation e.g. mTOR and GTF3C1 [27]. This indicates that SIRT7might also be important for noncatalytic roles in addition to itsdeacetylase activity. Thus as compared to other sirtuins (like SIRT3whose depletion leads to global hyperacetylation of mitochondrialproteins) SIRT7 seems to exhibit a weaker and rather a more substratespecific deacetylase activity. Also no other enzymatic activity (like ADPribosyltransferase activity) has been attributed to SIRT7. To date onlyp53, H3K18, PAF53, NPM1 and GABP-β1 are the known substrates forthe deacetylase activity of SIRT7 (Fig. 2). Variability in the acetylationstatus of p53 with respect to SIRT7 needs additional investigation.

Fig. 2. Catalytic substrates and in

Presently known substrates are insufficient to explain the major effectsof SIRT7 on various cellular processes such as those on rDNA transcrip-tion and ER stress. Hence additional substrates for SIRT7 deacetylaseactivity needs to be identified for better understanding of this protein.

4. Localization and nucleolar targeting of SIRT7

Most of the studies report SIRT7 to be primarily a nucleolar protein.Endogenous as well as GFP tagged SIRT7 showed nucleolar localization[9,28]. A deletion of the nucleolar localization signals in the full lengthGFPSIRT7 was sufficient to abrogate nucleolar localization of SIRT7[29]. SIRT7 achieves its nucleolar localization through two distinct local-ization signals rich in basic amino acids: a nuclear localization signallocated between 61 and 75 amino acids and a nucleolar localizationsignal located at the C-terminus between 392 and 400 amino acids(Fig. 1). Both these signals were found to be essential and sufficientfor the nucleolar targeting of SIRT7 [29].

While the nucleolar localization of SIRT7 is undebated its redistribu-tion to other compartments has been observed by various workers in acontext dependent manner. As the nucleolus gets degraded in themitotic stage of the cell cycle, the localization of SIRT7 in different stagesof mitosis has been studied. Initially, Ford et al. reported that SIRT7tagged to GFP remains excluded from nucleolar organizer regions(NORs) but stays associated with chromatin throughout the mitoticphase [28]. Later, Grob et al. [30] reported that although ectopicallyexpressed GFP tagged SIRT7 is excluded from theNORs, the endogenousSIRT7 remains associated with the NORs like other components of RNApolymerase complex. This association with NORs established closeassociation of SIRT7 with rDNA transcription [30].

The exclusively nucleolar niche of SIRT7 tagged to GFP was lost inconditions of glucose deficiency induced stress, when SIRT7 wasredistributed throughout the nucleoplasm [24]. A similar redistributionof SIRT7 tagged to GFP was also noted upon RNAse A or Actinomycin D(inhibitor of pre-rRNA synthesis) treatment indicating that nucleolarretention of SIRT7 is facilitated by the presence of RNA or ongoingrDNA transcription in the nucleolus [24,28]. In contrast, Tsai et al. [27]reported an RNA-independent nucleolar localization of SIRT7. They didnot find a redistribution of SIRT7 from nucleolus following RNAse Atreatment although they found it to happen for another nucleolar pro-tein Mybbp1a which is a RNA binding nucleolar protein [27]. Thus, thenucleolar localization of SIRT7 seems to be dependent on nuclear andnucleolar localization signals rather than on RNA binding. In additionto the presence of a 45 kDa nucleolar SIRT7, we also reported theexistence of a cytoplasmic (47.5 kDa) form [29]. The 47.5 kDa formof SIRT7 could be detected in all cell lines by immunoblotting butcould be detected only in fibroblast (but not in epithelial cell types)cells by immunofluorescence. Also 47.5 kDa form showed an inversecorrelation with the expression levels of the nucleolar form in differentstages of cell cycle and during replicative senescence [29]. Grob et al.[30] also demonstrated a phosphorylated state of SIRT7 migrating at ahigher molecular weight during the mitotic phase of cell cycle. SIRT7

teracting proteins of SIRT7.

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phosphorylation is dependent on the CDK1–cyclin B pathway and is de-phosphorylated at the exit frommitosis. These phosphorylated and de-phosphorylated forms could be differentiated on gel using a Phos-tagcompound [30].

Thus, like other sirtuins alternative localization sites have beenobserved for SIRT7 as well. The role of localization signals as well asRNA inmaintaining thenucleolar retentionof SIRT7 appears crucial. How-ever, the difference in the localization of endogenous SIRT7 and GFPSIRT7during the mitotic phase of cell cycle warrants a difference in the behav-iour of ectopically expressed SIRT7 to that of the endogenous form.

5. Expression pattern of SIRT7

SIRT7 mRNA is ubiquitously expressed in all tissues of mouse, al-though their expression levels varied. SIRT7 expression was low innonproliferating tissues like the muscle, heart and brain but high inmetabolically active tissues like the liver, spleen and testis [28]. Agedmice heart (23 months) showed about 40% decrease in the expressionlevels of SIRT7 [21]. Primary fibroblasts undergoing senescence showednot only a decrease in SIRT7 transcript levels but also a loss of nucleolarSIRT7 with replicative senescence in primary fibroblasts [9,29]. Howev-er primary human mammary epithelial cells showed an increase in theexpression of SIRT7 at transcript level with progressive passaging [31].The expression levels of SIRT7 were also found to increase during ERstress induced in response to thapsiagargin or tunicamycin treatment[32]. SIRT7 was found to be upregulated in thyroid, breast, bladderand hepatocellular cancer tissues [31,33–35]. Cancerous cell linesMCF-7, Saos-2 and A2780 specifically selected for chemoresistanceshowed a decreased expression of SIRT7 compared to the original non-resistant cell lines [36]. Hyperglycaemia caused an increased expressionlevel of nuclear sirtuins SIRT1 and 6 in mouse hepatocytes but not ofSIRT7 [37].

6. Role of SIRT7 in rDNA transcription

Synthesis of rRNA (or rDNA transcription) and assembly of nascentribosome is the major activity of nucleolus in the cell. Genes for rRNAare clustered on five different chromosomal tips which coalesce in thenucleolus [38]. RNA polymerase I (pol I) mediated transcription ofthese rDNA clusters followed by RNA processing leads to the synthesisof 4.5S, 18S and 28S rRNA which are then packaged with ribosomalproteins to form small and large subunits of the ribosome. Prominentlocalization of SIRT7 in the nucleolus suggested a role of SIRT7 inrDNA transcription. Accordingly SIRT7 has been associated to differentaspects of rDNA transcription in various studies. In the chromatin,SIRT7 lies closely associated with transcriptionally active rRNA genes[28]. The distribution of SIRT7 was found to be similar to that of RNApolymerase I and SIRT7 was found to be a part of the RNA polymeraseI complex. Inhibition of rDNA transcription with Actinomycin D treat-ment or degradation of RNA by RNAse A caused the redistribution ofectopically expressed SIRT7 from nucleolus to the nucleoplasm. Thisindicated that SIRT7 is closely linked to rDNA transcription in the nucle-olus. Overexpression of SIRT7 stimulated pol I mediated transcriptionand its knockdown dramatically reduced the expression levels of pre-rRNA [28]. Yet in another report, it has been shown that the knockdownof SIRT7 reduced the rate of rRNA synthesis by almost half [27].

Association of SIRT7 to RNA polymerase I complex is shown to bemediated through its interaction with UBF (upstream binding factor),a component of RNA polymerase complex essential for the formationof pre-initiation complex at the promoter site [30,39]. Sirtinol, a generalsirtuin inhibitor causes the inhibition of rDNA transcription [30]. Thesirtinol mediated inhibition of rDNA transcription was found to be spe-cific to SIRT7 activity as the knockdown of SIRT7 alone (but not SIRT1)inhibited rDNA transcription. Also rDNA transcription failed to restartafter mitosis in cells lacking SIRT7 [30].

SIRT7 downregulation was also found to decrease the expressionlevels of RNA polymerase I, by reducing the expression levels of ribo-somal protein, RPA194, the largest subunit of RNA polymerase I [40].In vitro transcription assays using cell extracts showed about 4 foldincrease in transcription upon the addition of NAD+ (0.8 mM). Thisincrease in transcription was not seen in extracts from SIRT7 depletedcell extracts indicating a specific role of SIRT7 in regulating RNApolymerase I mediated transcription [27].

SIRT7 can also influence rDNA transcription through the deacetylationof PAF53 at lysine 373 (Fig. 3A). PAF53 is a component of RNApolymeraseI complex which helps in the recruitment of polymerase I to rDNA pro-moters by interactingwithpol I subunit CAST/hPAF49 andUBF [41,42]. Ei-ther of the acetylation or deacetylation steps was important for pre-RNAsynthesis as the knockdown of either CBP or SIRT7 caused decreasedlevels of pre-rRNA synthesis. Cells with SIRT7 knockdown showeddecreased occupancy of PAF53 at transcribed regions. Redistribution ofnucleolar SIRT7 to nucleoplasm in conditions of stress impaired PAF53deacetylation, thereby inhibiting rDNA transcription [24].

7. SIRT7 and protein synthesis

Involvement of SIRT7 in protein synthesis was first reported by Kimet al. [35]. They found that the knockdown of SIRT7 in liver cancerouscells (Hep3B, SNU-368, SNU-449 and Huh7) decreased the expressionlevels of proteins ectopically expressed from different plasmids [35].Through affinity purification and mass spectrometry approaches SIRT7was found to be associated to ribosomal proteins and components ofTFIIIC2 complex. Enrichment of SIRT7 interactions in the nuclear fractionspulled down the components of RNA polymerase III (POLR3B, POLR1D,POLR3A, POLR1C, POLR3C, POLR3D, POLR2H and POLR2E) as well as thecomponents of TFIIIC2 complex (GTF3C1, GTF3C2, GTF3C3, GTF3C4,GTF3C5 and GTF3C6) [27]. This suggested a role of SIRT7 in protein syn-thesis, as ribosomes are the components of translational machinery andRNA polymerase III is involved in the synthesis of tRNAs and 5S rRNA. Ac-cordingly the amino acid incorporation rate was severely reduced (3 foldless) following SIRT7 knockdown.However, SIRT7 overexpression did notincrease the rate of protein synthesis indicating that reduced protein syn-thesiswas an indirect effect of SIRT7 knockdown. The reduced amino acidincorporation rates were found to be due to the decreased abundance oftRNAs for various amino acids following SIRT7 knockdown. Using ChIPbased assays, SIRT7 along with RNA polymerase III was found to be asso-ciated with genetic regions of tRNA and 5S rRNA [27]. However the ex-pression level of ribosomal proteins (RPS7 and RPS14) decreasedfollowing the increase in the expression level of SIRT7. This decrease inthe expression of ribosomal proteins was mediated at transcription levelthroughMycmediated recruitment of SIRT7 to the promoters of ribosom-al proteins, RPS7 and RPS14 [32].

The role of SIRT7 in rRNA and protein synthesis has highlighted therole of SIRT7 in stressful conditions. Regulation of rRNA and proteinsynthesis in conditions of cellular stress SIRT7 preserves precious cellu-lar energy and buys time for survival through the adverse conditions.Accordingly SIRT7 has been reported to help in cellular survival in anumber of stress conditions as discussed in the next section.

8. SIRT7 in cellular stress

Recent reports on SIRT7 have highlighted its role in stress resistance.SIRT7 has been shown to resist stress due to various adverse conditionssuch as hypoxia, ER-stress due to unfolded protein response, genomicinsult and nutrient stress due to low glucose (Fig. 3). The role of SIRT7in the control of rRNA as well as in protein synthesis further underlinesthe importance of SIRT7 in tolerating cellular stress. Both these process-es are halted or downregulated in conditions of cellular stress. The ac-tive role of SIRT7 in cellular survival under different conditions ofstress, are discussed below.

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8.1. Hypoxia induced stress

Hypoxia induced factors (HIF 1 and HIF 2) are transcription factorswhich activate a number of proteins during hypoxic conditions formounting an adaptive response. In normal conditions HIF proteins arebeing continuously ubiquitinylated through O2 dependent proline orasparagine hydroxylation [43,44]. SIRT7 was found to directly interactwith ectopically expressed HIF (HIF-1α and HIF-2α) proteins. However,the interaction of SIRT7 was weaker in comparison to another sirtuin,SIRT6 [45,46]. The protein level of ectopically expressed HIF proteinswas downregulated following the overexpression of SIRT7 in anumber of cell lines (Hep3B, HeLa, HEK293T and MDA-MB-231). SIRT7overexpression also downregulated the transcriptional targets ofHIF proteins like erythropoietin, superoxide dismutase and vascular

(A)

(B)

(C)

(E)

(D)

Fig. 3. The role of SIRT7 in cellular stress: (A) under normal conditions, SIRT7 maintains PAnucleoplasm leaves PAF53 in an acetylated condition through p300 mediated acetylation. Thpromotes transcriptional upregulation of SIRT7, which then collaborates with Myc to downredownregulating the expression of HIF 1α and HIF 1β as well as its transcriptional targets EPOand JNK) phosphorylation as well as the levels of γ-H2AX and p53. Overall SIRT7 helps in evadulatory effect on RNA polymerase I and RNA polymerase III is important in regulating rRNA an

endothelial growth factor (Fig. 3C). However, SIRT7mediated downreg-ulation of HIF proteins was independent of the catalytic activity of SIRT7as a catalytically dead form of SIRT7 also downregulated HIF levels. Thiswas also supported by the fact that nicotinamide (NAM), a general in-hibitor of sirtuin catalytic activity also had no effect on SIRT7 mediateddownregulation of HIF proteins as well. Additionally, SIRT7 mediateddownregulation of HIF proteins was independent of hydroxylation me-diated ubiquitinylation as well as lysosomal and proteasomal mediateddegradation of HIF proteins. Conversely, the knockdown of SIRT7 in-creased expression levels of ectopically expressed HIF proteins as wellas increased the transcriptional activities of HIF proteins in hypoxic con-ditions [46]. The role of other sirtuins viz. SIRT1 and SIRT6 in the regula-tion of activity as well as expression levels of HIF proteins have beenreported earlier [45,47]. Thus a growing body of evidence is supporting

F53 in a deacetylated state. Stress mediated redistribution of SIRT7 from nucleolus tois halts the ongoing transcription during conditions of stress. (B) During ER stress XBP1gulate the synthesis of ribosomal proteins. (C) SIRT7 relieves hypoxia induced stress by, SOD2 and VEGF. (D) During genomic stress SIRT7 specifically inhibits SAP kinase (p38ing cell death by promoting cellular survival in conditions of DNA damage. (E) SIRT7 reg-d protein synthesis in conditions of stress.

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the notion that sirtuins in general help in evading stressful cellular condi-tions. The fact that SIRT7 canmodulate HIF expression independent of itscatalytic activity exemplifies the fact that there are non-enzymaticmechanisms through which sirtuins can regulate downstream effectors.

8.2. Low glucose stress

Cellular processes in actively proliferating cells like ribosomebiogenesis are intrinsically linked to cellular energetics. AMPK is onesuch sensor molecule which shuts off rRNA as well as protein synthesisin low glucose conditions [48]. Similarly, SIRT7 was found to be impor-tant in uncoupling rRNA synthesis in low glucose conditions. Nutrientstress due to either low glucose or treatment with AICAR (low energymimic) shows the redistribution of exogenously expressed GFPSIRT7from nucleolus to nucleoplasm [24]. Under low energy conditions,RNA pol I mediated transcription was lost and this is accompanied bythe redistribution of nucleolar SIRT7 to nucleus. Mechanistically, SIRT7was found to be essential for maintaining a deacetylated state ofPAF53, an essential requirement for the recruitment of RNA polymeraseI to rDNA promoters (Fig. 3A). Redistribution of SIRT7 to the nucleoplasmabolished its interaction with PAF53, causing PAF53 hyperacetylationby CBP. The acetylated form of PAF53 abolished the recruitment of poly-merase I to rDNA promoters thereby terminating the process of rDNAtranscription [24]. The inhibition of rDNA transcription was also depen-dent on NAD+ concentration through SIRT7 activation. Thus SIRT7 wasvital (by promoting PAF53 acetylation) in sensing cellular energy levels(through cellular NAD+ levels) for shutting down rDNA transcription,an importantmechanism of conserving energy during stress [24]. Similarredistribution from nucleolus to nucleoplasm has been noted for severalnucleolar proteins like TIF1, nucleolin, and nucleophosmin during lowglucose stress [49–51]. SIRT7 therefore falls in the growing category ofnucleolar proteins which have ascribed nucleolus as a stress sensormachinery to modulate cellular processes for survival.

8.3. Endoplasmic reticulum stress (ER-stress)

Endoplasmic reticulum (ER) is the site of proper protein folding andsecretion. ER stress is induced in cells usually in response to the accu-mulation of unfolded proteins or when there is a depletion of calciumstores in the endoplasmic reticulum [52,53]. ER stress response ischaracterised by an adaptive reversible phase which is characterisedby increased levels of ER chaperones, degradation of unfolded proteinsand shutdown of ongoing mRNA translation. This set of initial adaptiveresponse proteins includes IRE1, PERK and ATF6 which drive theactivation of transcription factors XBP1, ATF4 and ATF6 to drive theexpression of effector proteins from the nucleus [54,55]. Shin et al.[32] elucidated a specific role of SIRT7 in alleviating ER stress. In condi-tions of ER stress, XBP1 led to transcriptional upregulation of SIRT7 toincrease the levels of SIRT7 protein [32]. This upregulation of SIRT7was observed in various cell lines upon the induction of ER stress bytunicamycin and thapsiagargin treatments (HepG2, Hepa 1–6 andMEF). SIRT7 upregulation in turn reduced the expressions of ER stressresponse proteins— CHOP, XBP1s and GRP78 (Fig. 3B).

Also SIRT7was instrumental in inhibiting ongoingmRNA translationduring ER stress by inhibiting the synthesis of ribosomal proteins(RPS20, RPS14) [32]. SIRT7 was found to be specifically recruited topromoters of ribosomal proteins for their transcriptional downregula-tion and this recruitment of SIRT7 was found to be mediated throughMyc protein. While Myc itself is responsible for transcriptional upregu-lation of ribosomal proteins it specifically interacted with and recruitedSIRT7, in conditions of ER stress, to the promoters of ribosomal proteinsto bring about the transcriptional downregulation of ribosomal proteins(Fig. 3B). Knockdown of Myc caused the depletion of SIRT7 from pro-moters of ribosomal proteins but not from other promoters (likeNME1) [32]. The role of Myc protein was indispensable for SIRT7 medi-ated alleviation of ER stress.

Thus SIRT7 is recruited specifically to the promoters of ribosomalproteins throughMyc during ER stress and to promoters of tumour sup-pressor genes through ELK4 for maintenance of cancerous state [22,32].The role of SIRT7 in controlling protein synthesis in general has beenobserved by others as well [27,35]. Thus in general, SIRT7 is importantin switching on adaptive responses in conditions of ER stress.

8.4. Genomic stress

Sirtuins in general acts as guardians of genome [56]. Yeast Sir2 aswell as mammalian SIRT1, 3 and 6 have been associated to differentaspects of maintenance of genomic integrity in adverse condi-tions [13,57–59]. The role of SIRT7 in maintenance of genomic integrityis not very well studied. However, it appears that SIRT7 can help the ge-nome stability through the regulation of p53 or by repairing the DNAdamage. SIRT7 knockout MEFs were found to have higher apoptosisrates compared to wild type controls [60]. The decreased survivabilityof SIRT7 knockouts was reasoned to be due to the presence ofhyperacetylated p53 in the absence of SIRT7. And this increased suscepti-bility to apoptosis was overcomewhen p53was inhibited by the additionof pifithrin [60]. A study which mainly focussed on SIRT6 additionallyshowed that SIRT7was found to increase theDNAdamage repair efficien-cy in paraquat treated cells [13]. SIRT7 overexpressing cells improved theefficiency of homologous recombination by 2.8 fold and nonhomologousend joining by 1.5 folds [13]. Recently we elaborated a protective role ofSIRT7 in conditions of genomic stress [61]. We reported that SIRT7 over-expression inNIH3T3 andU2OS cells protected these cells fromdoxorubi-cin induced senescence and apoptosis. Also, the knockdown of SIRT7sensitised these cells to doxorubicin induced apoptosis. Mechanisti-cally, SIRT7 overexpression was found to inhibit the activation of stressactivated protein kinases (SAPK), p38 and JNK in conditions of DNAdamage (Fig. 3D). The levels of γ-H2AX accumulation subsequent toDNA damage were also reduced in SIRT7 overexpressing cells. SIRT7also attenuated the levels of p53 accumulation subsequent toDNAdam-age and elevation of p53 levels with nutlin treatment alleviated SIRT7mediated protection from genomic stress [61].

9. SIRT7 and chromatin remodelling

Most of the sirtuins including SIRT7 modify chromatin by acting ashistonedeacetylases. SIRT7 in addition, associateswith anddeacetylatesa component of the B-WICH chromatin remodelling complex. B-WICHcomplex is associated with both RNA-pol I as well as polymerase IIIdependent transcriptions [62,63]. Components of the B-WICH complexare associated with each other through RNA. Using a number of proteo-mics and bioinformatics approaches, SIRT7 was found to be closelyassociated with the B-WICH complex [40]. Also SIRT7 interacted withMybbp1a an important component of the B-WICH complex andMybbp1a overexpression inhibited SIRT7 mediated H3K18 deacetylaseactivity [23]. Thus, SIRT7 being closely associated with the polymeraseI as well as the B-WICH complex, integrates rDNA transcription to chro-matin modification. SIRT7 knockdown was also found to downregulatethe expression of RPA194 protein (but not mRNA levels) which is thelargest component of RNA polymerase I complex. This downregulationof RPA194 was proposed to be due to the proteasomal degradation ofcomponents of stalled replication fork in the absence of SIRT7 [40].The effect of SIRT7 on protein synthesismight also bemediated throughits association with the B-WICH complex as this complex is associatedwith pol III mediated transcription as well. However this aspect needsfurther investigation.

10. .Regulation of SIRT7

MicroRNA miR-125a-5p and miR-125b were identified as endoge-nous regulators of SIRT7 [35]. SIRT7 was found to be regulated bymicroRNA hsa-miR-125b. The binding site for this has been identified

679S. Kiran et al. / Cellular Signalling 27 (2015) 673–682

in the 3 UTR region of SIRT7. The expression levels of hsa-miR-125bwere found to be downregulated in bladder cancer patients whereSIRT7 expression levels are high. Mimics of hsa-mir-125b downregulat-ed SIRT7 expression levels equivalent to knockdown levels caused bySIRT7 specific siRNA [34]. Another recently identified regulator ofSIRT7 is Mybbp1a. Mybbp1a was identified among SIRT7 interactingproteins [23,40]. C-terminal region of Mybbp1 protein was found tointeract with both the N- and C-terminus regions of SIRT7 protein.Mybp1a was found to inhibit the deacetylase activity of SIRT7, howeverthe exact mechanism of this inhibition was not elucidated [23].

11. .SIRT7 in disease and cancer

SIRT7 association with disease conditions has mainly come fromSIRT7mice knockout studies. SIRT7 knockout mice showed pathologiesassociatedwith cardiac and hepatic dysfunctions. Till date there are fourdifferent studies where SIRT7 knockout mice have been reported withdisease association (Table 1).

11.1. SIRT7 and cardiac health

The role of SIRT7 in maintenance of cardiac health was revealed inthe first SIRT7 knockout study by Vakhrusheva et al. [21]. Mice lackingSIRT7 had enlarged heart and they suffered from degenerative cardiachypertrophy. The hypertrophy was not visible in early life stages (upto 3 months) of mice but only from 7 months onwards, when a pro-nounced increase in heart weight was noticed. Also the cardiac cellsshowed an increased rate of apoptosis indicative of cardiac degenera-tion. Histological examination of the heart tissue revealed fibroticchanges in these mice as indicated by collagen III and collagen VI accu-mulation and intra-lysosomal lipofuscin deposition. There were alsoincreased inflammatory responses in these mice as evidenced byincreased number of T lymphocytes and granulocytes as well asincreased levels of interleukins 12 and13 in the blood of SIRT7 knockoutmice [21]. In a recent SIRT7 knockout study, these pathologies wererecapitulated in an independently generated mouse [26]. These micehad increased blood lactate levels and showed decreased enduranceto physical activity. This was attributed to cardiac muscle insufficiency.Increased cardiac lactate production could be attributed to decreasedoxygen consumption rate of cardiac muscles which in turn was due todecreased levels and activity of mitochondrial respiratory complexes:complex I and complex IV [26].

Table 1Summary of the SIRT7 knockout studies.

Study I Study II Study

Mouse strains C57BL/6 129Sv C57BLMajor phenotype Cardiac hypertrophy Hepatic steatosis without obesity.

Enlarged and paler liver.Resistasize.

Lifespan Reduced by 59% NR NRLipid metabolism NR Increased expression of lipogenic

genes. No change in fatty oxidationgenes.

Decrefatty o

Secretion rate ofVLDL lipoprotein

NR Decreased rate of VLDL secretion. No ch

Plasma triglyceridelevels

NR Decreased plasma triglyceridelevels.

No ch

Weight gain NR Decreased body weight. AttenuEpididymal WATweight

NR Decreased epididymal WAT weight. Decreadiet.

BAT weight NR NS DecreSub-cutaneous fat Loss of

sub-cutaneous fat.NR NR

Mechanisticexplanation

Constitutive activation of ER stressin the absence of SIRT7.

ConstiDCAF1degrad

Reference [21] [32] [64]

NR: not reported; WAT — white adipose tissue; BAT — brown adipose tissue.

11.2. SIRT7 in hepatic steatosis

Three recent independent mice knockout studies have pointedtowards a role of SIRT7 in hepatic lipid metabolism. While two of thestudies claimed that SIRT7 knockout led to hepatic steatosis in mice[26,32], another study reported that SIRT7 knockout mice were in factresistant to hepatic steatosis when fed on high fat diet [64]. Shin et al.in 129Sv SIRT7 knockout mice reported a hepatic degenerationcharacterised by enlarged and paler morphology of the liver [32].These mice had vacuolated hepatocytes with lipid accumulation similarto fatty liver disease as reported in humans. Therewas a 2.5 fold increasein the triglyceride content and increased accumulation of inflammatorymarkers in the hepatocytes of SIRT7 knockout mice. The increased fatcontent was due to the increased expression of lipogenic genes howeverthere was no change in the expression of fatty acid oxidation enzymes.Also therewas a reduced secretion of very low density lipoproteins lead-ing to an impaired transport of fat from the liver. However the knockoutmice were leaner in spite of fatty liver. Reconstitution of the hepaticSIRT7 in the knockout mice could reverse the fatty accumulation in theliver. Increased hepatic lipogenesis in SIRT7 knockout mice was attribut-ed to the constitutive activation of UPR (unfolded protein response) inthe absence of SIRT7. This indicated the importance of SIRT7 in thealleviation of ER stress [32].

In another study Yoshizawa et al. generated SIRT7 knockoutmice in C57BL/6 background and studied fatty acid metabolism[64]. The results from this study were in contrast to that of the pre-vious study. SIRT7 KO mice were resistant to weight gain andobesity compared to wild typemice when fed on high fat diet. In re-sponse to high fat diet the SIRT7 KOmice showed less accumulationof hepatic lipid (triacylglycerols and cholesterol) than wild typemice. These mice showed a decreased expression of genes involvedin triglyceride synthesis like monoacylglycerol O-acyltransferase 1(Mogat1), Cidea and Cidec. Also genes responsible for lipogenesiswere reduced in these mice. These hepatic changes were also pres-ent in mice, when SIRT7 was knocked out specifically from the miceliver. The KO mice also showed an improved glucose and insulintolerance and improved insulin sensitivity compared to wild typemice. SIRT7 KO mice liver was found to have decreased levels ofTR4/TAK1 nuclear receptor which is important in the regulationof lipid homeostasis.

Mechanistically, SIRT7 was found to protect TR4 a regulator ofhepatic lipid homeostasis from its proteasomal degradation. SIRT7 was

III Study IV

/6 C57BL/6nce to hepatic steatosis. Decreased liver Enlarged liver with microvesicular

hepatosteatosis.NR

ased expression of lipogenic as well asxidation genes (Mogat1).

No change in levels of lipogenic and fattyacid oxidation genes.

ange in the VLDL secretion rate. No change in genes involved in triglyceridesecretion.

ange. Increased plasma triglyceride levels.

ated weight gain on a high fat diet. No change in body weight.sed epididymal WAT weight on a high fat NR

ased BAT weight on a high fat diet. NRNR

tutive activation of ubiquitin ligase/DDB1/CUL4B leading to TR4ation.

Mitochondrial dysfunction due to GABP-β1hyperacetylation in the absence of SIRT7.

[26]

680 S. Kiran et al. / Cellular Signalling 27 (2015) 673–682

found to interact with and specifically inhibit the ubiquitinylase activityof the ubiquitinylation complex DCAF1/DDB1/CUL4B, responsible forTR4 degradation. Due to this a constant level of TR4 is maintained inthe liver whichmaintains lipid homeostasis. Loss of SIRT7 in the knock-out mice alleviated this inhibition on DCAF1/DDB1/CUL4B mediatedubiquitination, leading to the degradation of TR4. Decreased hepaticlipid accumulation in SIRT7 KO mouse was attributed to the loss ofTR4. Infact it has been noted that TR4 knockoutmice are resistant to he-patic steatosis when fed on a high fat diet [64,65]. The same group hadearlier proposed that SIRT7 promotes adipogenesis by binding to andinhibiting SIRT1 [66]. In the absence of SIRT7 therewas a reduced adipo-genesis in the liver and white adipose tissue (WAT) of SIRT7 knockoutmice. Knockdown of SIRT1 could reverse this phenotype as SIRT1 isknown to promote lipid mobilisation [67].

In support of the study by Shin et al., an independent study by Ryuet al. also found SIRT7 knockout mice to suffer from hepatic steatosis[26]. These mice had normal body weight but had heavier liver withmicrovesicular steatosis and had elevated levels of plasma triglyceridesand free fatty acids. However, in contrast with the first two studies, nochange in the expression levels of genes involved in ER stress, lipogen-esis, fatty acid oxidation, lipid uptake or storage and triglyceride secre-tion was observed. Instead, they found mitochondrial dysfunctionin liver to be responsible for this phenotype. The expression level ofSIRT7 was found to correlate with many mitochondrial proteins [26].Specifically, SIRT7 was found to interact with and deacetylate GABPβ1which is a nuclear transcription factor responsible for several mitochon-drial functions as well as its biogenesis. Acetylation of GABPβ1 reducesits capacity to hetero- or homo-dimers with another GABPβ1 orGABPα1 subunits. SIRT7 maintains GABPβ1 in a deacetylated stateto maintain its active form in normal cells. In SIRT7 knockout miceGABPβ1 acetylation levels were increased which inhibited its mitochon-drial transcriptional activities to cause mitochondrial dysfunction. Theexpression levels of ATP generating oxidative phosphorylation units(complex IV and II) as well as respiration through these complexeswere also reduced in the knockout mice. These hepatic pathologieswere also recapitulated in liver specific SIRT7 knockout mice [26].

Thus, SIRT7 knockout studies have definitely pointed towards a roleof SIRT7 in maintenance of hepatic lipid metabolism. However themechanisms proposed in these studies have failed to make a consensusabout the pathway through which SIRT7 exhibits its effects. Futurestudies should be directed towards understanding the variability inthese observations.

11.3. Role of SIRT7 in cancer

In general it has been difficult to ascribe sirtuins as an oncogene ortumour suppressor [68]. SIRT1 has been most extensively studied inthe context of cancer and its expression pattern has varied in differentcancer types [69–72]. In contrast, SIRT7 has been found to be upregulat-ed in all the cancer types studied so far such as thyroid cancer, node pos-itive breast cancer, bladder cancer, hepatocellular carcinoma andcolorectal cancer; and hence is seen as a potential oncogene [31,33–35,73]. Also many of these studies reported increased expressionlevels in cancerous cell lines compared to their normal counterpartand a knockdown in the expression levels of SIRT7 in these cell lines ab-rogated their proliferative or oncogenic capacity [22,35,73].

Work by Barber et al. [22] attributed SIRT7 in cancer to be importantand essential for the maintenance of cancerous phenotype. SIRT7 wasfound to be essential formaintaining deacetylated state of H3K18 at pro-moters of many tumour suppressor genes (Fig. 4A). Deacetylated stateof H3K18 has been associated with aggressive tumours with poorsurvival rates among patients and cancerous transformations are oftenassociated with the loss of H3K18 acetylation [74,75]. The SIRT7–ELK4connection appeared important for neoplastic progression. ELK4 knock-down led to a significant decrease of SIRT7 occupancy and increasedH3K18 acetylation at specific sites of genes such as the tumour suppressor

NME1. Also ELK4 knockdown alleviated SIRT7 overexpression mediatedrepression of tumour suppressor genes. Depletion of SIRT7 in cancerouscell lines, HT1080 and U2OS impaired their anchorage independence insoft agar assays and reduced cell proliferation in low serum conditions.E1A oncogene mediated cancerous transformation was also inhibited inthe absence of SIRT7 primarily due to the inability of H3K18 deacetylationin SIRT7 deficient cells. Tumour volumes formed by subcutaneousinjection of U251 cells lacking SIRT7 were also significantly reduced.Thus, SIRT7 was required for maintenance of all active oncogenic proper-ties of cancerous cells [22].

Kim et al. [35] reported an oncogenic role of SIRT7 in hepatocellularcarcinoma (HCC). SIRT7 expression levels were elevated in a largecohort of HCC patient samples as well as several hepatocellular cancer-ous cell lines. Knockdown of SIRT7 in the HCC cell lines inhibited theirgrowth rate by decreasing the expression levels of p21 and increasingthe levels of cyclin D1 thereby leading to a decreased G1/S progression.The increased expression of SIRT7 in HCC patients was due to theendogenous hypermethylation of microRNAs, miR-125a-5p and miR-125b (Fig. 4B). Removal of methylation marks with aza-cytidine treat-ment restored endogenous expression of these microRNAs [35]. Therole ofmiR-125b in regulating SIRT7 expression levelswas also reportedin bladder cancer [34]. Bladder urothelial carcinomas showed a down-regulation of hsa-miR-125b accompanied with an increase in the ex-pression levels of SIRT7 and another oncogene MALAT1. Bladdercancer cell lines T24 and 5637 showed elevated expression levels ofSIRT7 and decreased levels of miR-125b. Upon SIRT7 knockdownthese cell lines also showed decreased cell proliferation rates as wellas cell migration, similar to other cancer cell types [34].

A recent study found similar results in colorectal cancer samples(CRCs) [73]. An increased expression of SIRT7 was found in colorectalcancer tissues as well as CRC cell lines (HCT116, DLD1, THC8307, HT29and SW620). Overexpression of SIRT7 in CRC cell lines increased theirgrowth rates and invasiveness in vitro as well as in vivo systems whilethe knockdown of SIRT7 reversed these phenotypes. In CRC patients,SIRT7 overexpression was associated with poor prognosis. The increasein growth rate in SIRT7 overexpressing cells was attributed to increasedlevels of p-ERK1/2 and cyclin D1 levels upon SIRT7 expression. SIRT7overexpressing cells also showed increasedmotilitywhichwas attribut-ed to the induction of mesenchymal characters in these cell lines. SIRT7overexpression decreased E-cadherin and β-catenin expression leadingto mesenchymal transition and increase cell motility [73] (Fig. 4C). Astudy in head and neck squamous cell carcinomas, reported a decreasein expression levels of all sirtuins including SIRT7 at transcript levels[76].

Studies on other sirtuins, mainly SIRT1 have not been successfulin assigning sirtuin as tumour promoters or tumour suppressors. Incontrast, a number of cancer studies have unanimously ascribed an on-cogenic role of SIRT7 in cancer. This may be due to the fact that SIRT7 isvital on one hand for maintenance of oncogenic properties throughH3K18 deacetylation and on the other hand its enhanced activity is re-quired in rDNA transcription tomeet the increased demands of ribosomesynthesis in faster proliferating cancer cells. SIRT7 by itself does not seempotent in imparting oncogenicity, as the overexpression of SIRT7 alonedid not cause oncogenic transformation of primary fibroblasts [22].Thus increased expression of SIRT7 in cancerous cells is most likely tomeet the demands of increased ribosomal rRNAaswell as ribosomal pro-tein biosynthesis in highly proliferating cancerous cells. Whether SIRT7overexpressionwill also act to promote onset of tumorigenesis in a back-ground of other oncogene (like mutant Ras) remains to be explored.However, from the existing reports it appears that SIRT7 can act as a tu-mour promoter to maintain the transformed cellular state (Fig.4).

12. SIRT7 in ageing and senescence

The founding member of sirtuin family Sir2 was initially identifiedin a screen for long lived yeast mutants. As such mammalian sirtuins

A)

B) C)

Fig. 4. The role of SIRT7 in cancer progression: (A) SIRT7 acts as a specific H3K18 deacetylase. SIRT7 is recruited through ELK4 to promoters of tumour suppressor genes to downregulate theirexpression and promote oncogenicity. On the other hand Myc mediated recruitment of SIRT7 to promoters of ribosomal proteins relieves ER stress which might be important for cancer cellsurvival in conditions of stress. (B) Hepatocellular as well as bladder cancer progression is accompanied with SIRT7 upregulation through silencing of microRNAs responsible for endogenousregulation of SIRT7. (C) Upregulated SIRT7 propels towards oncogenicity by ERK activation as well as promoting Epithelial to Mesenchymal transition (EMT).

681S. Kiran et al. / Cellular Signalling 27 (2015) 673–682

are also thought to be involved in the ageing process. However none ofthe sirtuins except SIRT6 have shown to directly resist or control ageingin the mammalian system. SIRT7 knockout mice showed almost 60%decrease in the mean lifespan while a decrease of 55% in the maximallifespan. The decreased lifespan was primarily attributed to heartdefects in the SIRT7 knockout mice [21]. Lower levels of SIRT7 expres-sion levels were reported in hepatocytes of aged mice (56 week oldBALB/cAnUnib mice) as well as aged rats (24 weeks old) compared totheir young counterparts [25,26,37]. Also in the recent study by Ryuet al. SIRT7 knockout mice found a loss of hearing ability in aged mice[26]. These mice showed increased threshold for auditory brain re-sponse (ABR) at all audible frequencies [26].

In cellular studieswith respect to senescence a decreased expressionof the nucleolar SIRT7 has been reported in fibroblasts [9,29]. Inductionof camptothecin mediated senescence in HEK293FT cells led to adecrease in the expression levels of SIRT7. Decreased levels of SIRT7during senescence led to increased acetylated forms of NPM1, knownto promote cellular senescence [25].

These studies indicated that in general decreased levels of SIRT7 waslinked to the onset of senescence or ageing and loss of SIRT7 appears tobe associated with a decreased life or health span. However, it remainsto be seen if SIRT7 overexpression can delay the onset of senescence orageing process. It is expected that through its potential to withstand cel-lular stress, cells with an increased expression of SIRT7 will be betterequipped to survive stressful conditions which usually prevail duringthe process of ageing.

13. Concluding remarks

SIRT7 for long has been eluded of the attention which other mam-malian sirtuins have received in the initial years of sirtuin research.This most probably was due to a weak catalytic (deacetylase or ADPribosyltransferase activity) as well as its low homology to yeast Sir2proteins in comparison to other human sirtuins. However, consistentefforts by various workers in recent reports have brought significantclarity to SIRT7 biology. The role of SIRT7 in rDNA transcription andits upregulation in different types of cancer have been univocallyestablished by various workers. Additionally, recent reports haveaccredited SIRT7 to the role of a stress buster for supporting cell sur-vivability in conditions of different types of stress. Due to its controlover rRNA as well as protein synthesis in general as well as a specificrole in stress conditions (through molecules such as HIF andIRE1alpha), SIRT7 emerges as an important stress adaptor moleculefor cellular survival. SIRT7 can now be seen as a pro-survival mole-cule in conditions of stress.

Overexpression of SIRT7 in cancerous tissues might also be seen asa survival mechanism of cancerous cells in hypoxic and low glucoseconditions, especially in solid tumours where oxygen and glucosesupplies are often limited. However an experimental evidence forthis role of SIRT7 in tumours is still awaited. At present very less isknown about the catalytic repertoire of SIRT7. Although global prote-omics studies have identified a number of SIRT7 interacting proteins,catalytic activity of SIRT7 on these proteins has not been studied.

682 S. Kiran et al. / Cellular Signalling 27 (2015) 673–682

Future studies are required for the identification of SIRT7 catalytictargets among its interacting proteins.

Unlike other sirtuins, studies on SIRT7 knockout mice have demon-strated the importance of this protein at organismal level. However, thecontrast in the phenotype of these studies was far more than could beexplained by difference in the strain and method by its knockout micewere generated. Therefore more of such knockout studies are requiredto arrive upon a consensus role of SIRT7 at organismal level.

Overall the recent understanding of SIRT7 has outlined a number ofcellular processes where SIRT7 is involved and added new dimensionsto the importance of this relatively obscure nucleolar protein. Thesurge of reports on SIRT7 in the last two years have greatly widenedthe understanding of SIRT7 and future studies will be directed towardsamore specific role of SIRT7 and its catalytic activity at cellular aswell asorganismal levels.

Acknowledgements

SK is supported by a fellowship from the Department of Biotechnol-ogy, India. TA and MK are supported by fellowships from UniversityGrants Commission, India and CSIR, India respectively. TA and MK areregistered for PhD programme in Manipal University, Karnataka. Labo-ratory of GR is supported by funds from the Department of Biotechnol-ogy, India.

References

[1] M. Kaeberlein, M. McVey, L. Guarente, Genes Dev. 13 (1999) 2570–2580.[2] B. Rogina, S.L. Helfand, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 15998–16003.[3] H.A. Tissenbaum, L. Guarente, Nature 410 (2001) 227–230.[4] Y. Kanfi, V. Peshti, R. Gil, S. Naiman, L. Nahum, E. Levin, N. Kronfeld-Schor, H.Y.

Cohen, Aging Cell 9 (2010) 162–173.[5] G. Kelly, Altern. Med. Rev. 15 (2010) 245–263.[6] G.S. Kelly, Altern. Med. Rev. 15 (2010) 313–328.[7] Y. Yao, Y. Yang, W.G. Zhu, Curr. Pharm. Des. 20 (2014) 1614–1624.[8] W. Giblin, M.E. Skinner, D.B. Lombard, Trends Genet. 30 (7) (Jul. 2014) 271–286,

http://dx.doi.org/10.1016/j.tig.2014.04.007 (Epub 2014 May 28., 2014).[9] E. Michishita, J.Y. Park, J.M. Burneskis, J.C. Barrett, I. Horikawa, Mol. Biol. Cell 16

(2005) 4623–4635.[10] P. Onyango, I. Celic, J.M. McCaffery, J.D. Boeke, A.P. Feinberg, Proc. Natl. Acad. Sci. U.

S. A. 99 (2002) 13653–13658.[11] B. Schwer, B.J. North, R.A. Frye, M. Ott, E. Verdin, J. Cell Biol. 158 (2002) 647–657.[12] D.B. Lombard, F.W. Alt, H.L. Cheng, J. Bunkenborg, R.S. Streeper, R. Mostoslavsky, J. Kim,

G. Yancopoulos, D. Valenzuela, A.Murphy, Y. Yang, Y. Chen,M.D.Hirschey, R.T. Bronson,M. Haigis, L.P. Guarente, R.V. Farese Jr., S.Weissman, E. Verdin, B. Schwer,Mol. Cell. Biol.27 (2007) 8807–8814.

[13] Z. Mao, C. Hine, X. Tian, M. Van Meter, M. Au, A. Vaidya, A. Seluanov, V. Gorbunova,Science 332 (2011) 1443–1446.

[14] Z. Mao, X. Tian, M. Van Meter, Z. Ke, V. Gorbunova, A. Seluanov, Proc. Natl. Acad. Sci.U. S. A. 109 (2012) 11800–11805.

[15] R.A. McCord, E. Michishita, T. Hong, E. Berber, L.D. Boxer, R. Kusumoto, S. Guan, X.Shi, O. Gozani, A.L. Burlingame, V.A. Bohr, K.F. Chua, Aging 1 (2009) 109–121.

[16] G. Donmez, T.F. Outeiro, EMBO Mol. Med. 5 (2013) 344–352.[17] M.C. Haigis, R. Mostoslavsky, K.M. Haigis, K. Fahie, D.C. Christodoulou, A.J. Murphy,

D.M. Valenzuela, G.D. Yancopoulos, M. Karow, G. Blander, C. Wolberger, T.A. Prolla,R. Weindruch, F.W. Alt, L. Guarente, Cell 126 (2006) 941–954.

[18] C. Schlicker, M. Gertz, P. Papatheodorou, B. Kachholz, C.F. Becker, C. Steegborn, J.Mol. Biol. 382 (2008) 790–801.

[19] T. Nakagawa, D.J. Lomb, M.C. Haigis, L. Guarente, Cell. 137 (2009) 560–570.[20] S. Voelter-Mahlknecht, S. Letzel, U. Mahlknecht, Int. J. Oncol. 28 (2006) 899–908.[21] O. Vakhrusheva, C. Smolka, P. Gajawada, S. Kostin, T. Boettger, T. Kubin, T. Braun, E.

Bober, Circ. Res. 102 (2008) 703–710.[22] M.F. Barber, E. Michishita-Kioi, Y. Xi, L. Tasselli, M. Kioi, Z. Moqtaderi, R.I. Tennen, S.

Paredes, N.L. Young, K. Chen, K. Struhl, B.A. Garcia, O. Gozani, W. Li, K.F. Chua, Nature487 (2012) 114–118.

[23] M.F. Karim, T. Yoshizawa, Y. Sato, T. Sawa, K. Tomizawa, T. Akaike, K. Yamagata,Biochem. Biophys. Res. Commun. 441 (2013) 157–163.

[24] S. Chen, J. Seiler, M. Santiago-Reichelt, K. Felbel, I. Grummt, R. Voit, Mol. Cell 52(2013) 303–313.

[25] N. Lee, D.K. Kim, E.S. Kim, S.J. Park, J.H. Kwon, J. Shin, S.M. Park, Y.H. Moon, H.J. Wang,Y.S. Gho, K.Y. Choi, Proteomics 29 (2014) 201400001.

[26] D. Ryu, Y.S. Jo, G. Lo Sasso, S. Stein, H. Zhang, A. Perino, J.U. Lee, M. Zeviani, R.Romand, M.O. Hottiger, K. Schoonjans, J. Auwerx, Cell Metab. 20 (2014) 856–869.

[27] Y.C. Tsai, T.M. Greco, I.M. Cristea, Mol. Cell. Proteomics 10 (2013) 10.[28] E. Ford, R. Voit, G. Liszt, C. Magin, I. Grummt, L. Guarente, Genes Dev. 20 (2006)

1075–1080.[29] S. Kiran, N. Chatterjee, S. Singh, S.C. Kaul, R. Wadhwa, G. Ramakrishna, FEBS J. 280

(2013) 3451–3466.

[30] A. Grob, P. Roussel, J.E. Wright, B. McStay, D. Hernandez-Verdun, V. Sirri, J. Cell Sci.122 (2009) 489–498.

[31] N. Ashraf, S. Zino, A. Macintyre, D. Kingsmore, A.P. Payne, W.D. George, P.G. Shiels,Br. J. Cancer 95 (2006) 1056–1061.

[32] J. Shin, M. He, Y. Liu, S. Paredes, L. Villanova, K. Brown, X. Qiu, N. Nabavi, M. Mohrin, K.Wojnoonski, P. Li, H.L. Cheng, A.J. Murphy, D.M. Valenzuela, H. Luo, P. Kapahi, R. Krauss,R. Mostoslavsky, G.D. Yancopoulos, F.W. Alt, K.F. Chua, D. Chen, Cell Rep. 5 (2013)654–665.

[33] F. De Nigris, J. Cerutti, C. Morelli, D. Califano, L. Chiariotti, G. Viglietto, G. Santelli, A.Fusco, Br. J. Cancer 87 (12) (2002) 1479 (2).

[34] Y. Han, Y. Liu, H. Zhang, T. Wang, R. Diao, Z. Jiang, Y. Gui, Z. Cai, FEBS Lett. 587 (2013)3875–3882.

[35] J.K. Kim, J.H. Noh, K.H. Jung, J.W. Eun, H.J. Bae, M.G. Kim, Y.G. Chang, Q. Shen, W.S.Park, J.Y. Lee, J. Borlak, S.W. Nam, Hepatology 57 (2013) 1055–1067.

[36] A. Aljada, A.M. Saleh, S. Al Suwaidan, Diagn. Pathol. 9 (2014) 94.[37] F.G. Ghiraldini, A.C. Crispim, M.L. Mello, Mol. Biol. Cell 24 (2013) 2467–2476.[38] I. Raska, P.J. Shaw, D. Cmarko, Int. Rev. Cytol. 255 (2006) 177–235.[39] S.P. Bell, R.M. Learned, H.M. Jantzen, R. Tjian, Science 241 (1988) 1192–1197.[40] Y.C. Tsai, T.M. Greco, A. Boonmee, Y. Miteva, I.M. Cristea, Mol. Cell. Proteomics 11

(2012) 60–76.[41] K. Hanada, C.Z. Song, K. Yamamoto, K. Yano, Y. Maeda, K. Yamaguchi, M.Muramatsu,

EMBO J. 15 (1996) 2217–2226.[42] K.I. Panov, T.B. Panova, O. Gadal, K. Nishiyama, T. Saito, J. Russell, J.C. Zomerdijk, Mol.

Cell. Biol. 26 (2006) 5436–5448.[43] M. Ivan, K. Kondo, H. Yang, W. Kim, J. Valiando, M. Ohh, A. Salic, J.M. Asara, W.S.

Lane, W.G. Kaelin Jr., Science 292 (2001) 464–468.[44] D. Lando, D.J. Peet, D.A. Whelan, J.J. Gorman, M.L. Whitelaw, Science 295 (2002)

858–861.[45] L. Zhong, A. D'Urso, D. Toiber, C. Sebastian, R.E. Henry, D.D. Vadysirisack, A.

Guimaraes, B. Marinelli, J.D. Wikstrom, T. Nir, C.B. Clish, B. Vaitheesvaran, O.Iliopoulos, I. Kurland, Y. Dor, R. Weissleder, O.S. Shirihai, L.W. Ellisen, J.M.Espinosa, R. Mostoslavsky, Cell 140 (2010) 280–293.

[46] M.E. Hubbi, H. Hu, D.M. Gilkes Kshitiz, G.L. Semenza, J. Biol. Chem. 288 (2013)20768–20775.

[47] E.M. Dioum, R. Chen, M.S. Alexander, Q. Zhang, R.T. Hogg, R.D. Gerard, J.A. Garcia,Science 324 (2009) 1289–1293.

[48] D.G. Hardie, Nat. Rev. Mol. Cell Biol. 8 (2007) 774–785.[49] C. Mayer, H. Bierhoff, I. Grummt, Genes Dev. 19 (2005) 933–941.[50] Y. Daniely, D.D. Dimitrova, J.A. Borowiec, Mol. Cell. Biol. 22 (2002) 6014–6022.[51] S. Kurki, K. Peltonen, L. Latonen, T.M. Kiviharju, P.M. Ojala, D. Meek, M. Laiho, Cancer

Cell 5 (2004) 465–475.[52] Y. Kozutsumi, M. Segal, K. Normington, M.J. Gething, J. Sambrook, Nature 332 (1988)

462–464.[53] J.B. DuRose, A.B. Tam, M. Niwa, Mol. Biol. Cell 17 (2006) 3095–3107.[54] C. Hetz, Nat. Rev. Mol. Cell Biol. 13 (2012) 89–102.[55] C. Xu, B. Bailly-Maitre, J.C. Reed, J. Clin. Invest. 115 (2005) 2656–2664.[56] L. Bosch-Presegue, A. Vaquero, Oncogene 33 (2014) 3764–3775.[57] K.D. Mills, D.A. Sinclair, L. Guarente, Cell 97 (1999) 609–620.[58] Z. Yuan, X. Zhang, N. Sengupta, W.S. Lane, E. Seto, Mol. Cell 27 (2007) 149–162.[59] N.R. Sundaresan, S.A. Samant, V.B. Pillai, S.B. Rajamohan, M.P. Gupta, Mol. Cell. Biol.

28 (2008) 6384–6401.[60] O. Vakhrusheva, D. Braeuer, Z. Liu, T. Braun, E. Bober, J. Physiol. Pharmacol. 9 (2008)

201–212.[61] S. Kiran, V. Oddi, G. Ramakrishna, Exp. Cell Res. (2014), http://dx.doi.org/10.1016/j.

yexcr.2014.11.001.[62] E. Cavellan, P. Asp, P. Percipalle, A.K. Farrants, J. Biol. Chem. 281 (2006)

16264–16271.[63] P. Percipalle, N. Fomproix, E. Cavellan, R. Voit, G. Reimer, T. Kruger, J. Thyberg, U.

Scheer, I. Grummt, A.K. Farrants, EMBO Rep. 7 (2006) 525–530.[64] T. Yoshizawa,M.F. Karim, Y. Sato, T. Senokuchi, K. Miyata, T. Fukuda, C. Go, M. Tasaki, K.

Uchimura, T. Kadomatsu, Z. Tian, C. Smolka, T. Sawa,M. Takeya, K. Tomizawa, Y. Ando, E.Araki, T. Akaike, T. Braun, Y. Oike, E. Bober, K. Yamagata, CellMetab. 19 (2014) 712–721.

[65] H.S. Kang, K. Okamoto, Y.S. Kim, Y. Takeda, C.D. Bortner, H. Dang, T. Wada, W. Xie, X.P. Yang, G. Liao, A.M. Jetten, Nuclear orphan receptor TAK1/TR4-deficient mice areprotected against obesity-linked inflammation, hepatic steatosis, and insulin resis-tance, Diabetes 60 (2011) 177–188.

[66] E. Bober, J. Fang, C. Smolka, A. Ianni, O. Vakhrusheva, M. Kruger, T. Braun, BMC Proc.6 (2012) 57.

[67] F. Picard, M. Kurtev, N. Chung, A. Topark-Ngarm, T. Senawong, R. Machado DeOliveira, M. Leid, M.W. McBurney, L. Guarente, Nature 429 (2004) 771–776.

[68] L. Bosch-Presegue, A. Vaquero, Genes Cancer 2 (2011) 648–662.[69] E.J. Cha, S.J. Noh, K.S. Kwon, C.Y. Kim, B.H. Park, H.S. Park, H. Lee, M.J. Chung, M.J.

Kang, D.G. Lee, W.S. Moon, K.Y. Jang, Clin. Cancer Res. 15 (2009) 4453–4459.[70] B. Jung-Hynes, M. Nihal, W. Zhong, N. Ahmad, J. Biol. Chem. 284 (2009) 3823–3832.[71] R. Firestein, G. Blander, S. Michan, P. Oberdoerffer, S. Ogino, J. Campbell, A.

Bhimavarapu, S. Luikenhuis, R. de Cabo, C. Fuchs, W.C. Hahn, L.P. Guarente, D.A.Sinclair, PLoS One 3 (2008) e2020.

[72] J. Yuan, K. Minter-Dykhouse, Z. Lou, J. Cell Biol. 185 (2009) 203–211.[73] H. Yu, W. Ye, J. Wu, X. Meng, R.Y. Liu, X. Ying, Y. Zhou, H. Wang, C. Pan, W. Huang,

Clin. Cancer Res. 25 (2014) 25.[74] G.A. Horwitz, K. Zhang, M.A. McBrian, M. Grunstein, S.K. Kurdistani, A.J. Berk, Science

321 (2008) 1084–1085.[75] D.B. Seligson, S. Horvath, M.A. McBrian, V. Mah, H. Yu, S. Tze, Q. Wang, D. Chia, L.

Goodglick, S.K. Kurdistani, Am. J. Pathol. 174 (2009) 1619–1628.[76] C.C. Lai, P.M. Lin, S.F. Lin, C.H. Hsu, H.C. Lin, M.L. Hu, C.M. Hsu, M.Y. Yang, Tumour

Biol. 34 (2013) 1847–1854.